A major problem in automatic weapon design lies
in the mechanism for inserting cartridges into the gun chamber and
removing the fired cases. In virtually all guns to see service, the
cartridge is pushed forwards into the rear of the chamber, and the
empty case pulled back out again. There is just one example of a
variation on this, the Russian Rikhter R-23 revolver cannon, in which
the process is reversed and the cartridges are pushed backwards into
the chambers from in front.

This design feature causes problems because a
high rate of fire requires the cartridges to be pushed and pulled
violently to and fro. The mechanisms used to achieve this have to make
large - and constantly reversing - movements in order to handle the
cartridges. This exerts considerable stress on the weapon, requiring it
to be carefully designed and strongly built. Even so, ammunition
handling problems are by far the greatest cause of gun unreliability.

REVOLVERS

Various attempts have been made to ease this
problem. One of them is the revolver cannon, which has several chambers
in a cylinder, rotating to place each chamber in turn in line with the
barrel. This enables the cartridges to be loaded and unloaded in
stages, slowing down the ammunition handling process. Modern revolver
cannon were first designed in Germany in World War 2, and have proved
highly successful in aircraft guns since the mid-1950s. Examples
include the British Aden and French DEFA/GIAT range, the American M39,
the Swiss Oerlikon KCA and the German Mauser BK 27. One oddity was the
USN's Mk 11, which had two barrels.

Revolver cannon have their own design problems,
however. Their action is not smooth as the rotation of the cylinder has
to be stopped for each shot, then started again. The gap between the
chamber and the barrel has to be bridged somehow to prevent gas
escaping. Finally, even if the gun works perfectly, the high rate of
fire rapidly erodes the barrel.

ROTARIES

These difficulties explain the popularity (albeit
mainly in the USA) of the rotary cannon, popularly known as the
"Gatling" after the inventor of the first successful gun of this type.
It is rather like the revolver cannon except that each chamber has its
own barrel permanently attached and rotating with it. This eliminates
the problem of the chamber/barrel gap, results in a much smoother
action as the gun spins constantly when firing, and solves the barrel
wear problem. The disadvantage (there are always disadvantages…) is
that the rotary is very bulky and heavy by comparison with other types,
and (especially when power driven) takes time to accelerate up to its
maximum firing rate, which can be a disadvantage in aerial combat. By
far the best known rotary gun is the American M61 "Vulcan", but various
other types are made; not just in the USA but also in Russia and
Switzerland.

THE TROUND

Unsurprisingly, much ingenuity has been devoted
to finding better ways of chambering and extracting the cartridges. One
of the more successful attempts was the Dardick "Tround", which
utilised a strong plastic cartridge of triangular section. The gun was
a revolver, but the chambers were open on the outside. The cylinder was
encased in a strong sleeve, except where the cartridge was loaded and
the case ejected. The operation was very simple; the cartridge was
dropped straight into the chamber through the gap in the sleeve, was
rotated in line with the barrel and when fired was supported by both
the cylinder and the outer sleeve, which combined to form the chamber.
A pistol using this system went on commercial sale for a while, and
machine guns and cannon were experimented with for many years, but not
adopted.

The principle of opening up the chamber to permit
sideways loading has some obvious advantages. The cartridges no longer
have to be pushed and pulled but drop straight in and out again. This
simplicity enabled very high rates of fire to be achieved, albeit with
the same problems as other types of revolver cannon. The added problem
is in ensuring that the cartridge is fully supported on firing, as
otherwise the case will split under the pressure. A drawing showing the
operating principle, taken from Chinn Vol. 5, is shown below.

THE NUTCRACKER

An alternative approach was the subject of
experiments in the UK in the 1950s, as part of research into future
aircraft guns. It was a split-breech gun, in which two cylinders, each
with several deep semi-cylindrical grooves along their outsides, were
held side by side and rotated in opposite directions. The rotation was
so arranged that grooves from each cylinder came together to form a
perfect cylindrical shape; the gun's chamber. The cartridges were
dropped between the rotating cylinders and collected by the grooves
which, for an instant, formed an all-enveloping chamber, at which point
the cartridge was fired. This action led to the nickname of
"Nutcracker".

The advantages of the Nutcracker design were
smoothness and simplicity of operation, with the potential for an
extremely high rate of fire. The disadvantages should also be apparent.
Each chamber was only formed for a brief instant as the cylinders
rotated; any mistiming in a cartridge firing would leave it partly
unsupported, leading to splitting along its length. There would also
have been the usual revolver problems of sealing the gap to the barrel,
and excessive barrel wear.

As far as this author knows, no written material
concerning these experiments is available (if anyone knows more, please email
me!). However, it is probable that the designers needed to stop the rotation at
the instant of firing, partly to ensure that the cartridge was supported and
partly to keep it lined up with the barrel until the projectile had completed
the transfer from the cartridge case to the barrel. This would have slowed down
the rate of fire and spoiled the smoothness of the action. The temptation must
have been to minimise any such interruptions in the interests of a high rate of
fire, but as the experiments failed it must be presumed that this dilemma was
never satisfactorily resolved: reportedly case splits were not uncommon.

The only surviving evidence of the
Nutcracker appears to be a few rounds of ammunition. These consist of plain
aluminium alloy tubes, within which the projectile is concealed. The
specimen examined by the author (courtesy of Steve McGregor) was of 30mm
calibre and the case was 330mm long. Views of it are shown below.
A photo of the sectioned cartridge (courtesy Geoff Tillotson) is shown on
the right.
Various other calibres were considered if not made, however, as shown in the
following table from ADE D2
Group Progress Note 1/53:

Assessment
of Future Weapons for RAF Fighter Aircraft (extract)

Calibre (mm)

Muzzle velocity
(m/s)

Case length
(mm)

Case weight (g)

HE content (g)

21.1

610

152.4

136

20

23.24

1065

246.4

272

20

23.24

1520

439.4

372

20

32

610

205.7

499

70

35.2

1065

675.6

817

70

35.2

1520

378.5

1180

70

43

610

292.1

1153

170

47.3

1065

487.7

1920

170

47.3

1520

878.8

2769

170

63.3

610

441.9

3632

544

70

1065

711.2

6356

544

THE 40MM MK 18

Despite this failure, there has been one
technically successful example of a split-breech gun; the American
Honeywell 40mm Mk 18 Mod 0 Multiple Grenade Launcher. It was the first
attempt to produce a fast-firing weapon to supplement the single-shot
M79 grenade launcher. It used the original low-velocity 40x46SR
ammunition (unlike later automatic grenade launchers, which use a
40x53SR round generating a much higher velocity). Most unusually, the
mechanism was manually-cranked using a handle on the side of the gun,
so the rate of fire depended on how quickly the gunner could turn the
handle. Design commenced in 1962 and approximately 1200 were built
between 1965 and 1968.

The design was basically the same as the
Nutcracker, but with two important differences which allowed it to
succeed. First, the rate of fire was kept very low (a maximum of around
250 rpm), and second, the cartridge fired was a very low-pressure one.
Unusually, the rounds were held in a belt made of fibreglass tape
(normally available in 25-round lengths) and they remained in the belt
as they were passed through the mechanism and fired, further
simplifying the design.

A drawing of the design layout (from Chinn, The Machine Gun, Volume 5)
is shown on the right.

It
will immediately be seen that the cylinders (or rotors, as they were
known) could not rotate from the firing position unless they were moved
away from each other; they then had to move back together again for the
next shot. It will also be evident that the same problem would have
affected the Nutcracker, complicating its operation. However, the Mk 18
remained a light and simple weapon, weighing just 8.6 kg and measuring
56 cm long.

A photo of the gun is shown below.

The Mk 18 was soon supplanted by the Philco-Ford
M75 Grenade Launcher, which was power driven as well as firing the
high-velocity cartridge. Despite this, the Mk 18 has a small place in
history as a very rare example of a different approach to the problems
of automatic weapon design.

Finally, it should be noted that the split-breech
approach was the subject of a US Patent in 1861 (#32,316), but does not
seem to have been followed up at the time. However, towards the end of
the First World War Fokker produced an engine-driven 12-barrel aircraft
gun. Apparently this was not a Gatling derivative but a 'nutcracker'
split-breech design, half of the chambers being attached to the
barrels, the other half incorporated in a cylinder under the barrels.
As with the Mk 18, the ammunition belt ran between the two cylinders,
with no need to remove cartridges from the belt. Firing, apparently,
was by a lever that was actuated by a cam on the drive wheel of the
gun. Fokker claimed that 7,200 rpm was achieved, but knowing Fokker,
there is some reason to assume that that may have been slightly
exaggerated. Problems occurred, of course, with cases bursting on the
seam between the two cylinders. The use of special cartridges (no
details known) was not acceptable in wartime, so Fokker's technicians
came up with two alternative solutions: Push the lower cylinder against
the upper one at firing time by means of strong springs; or give the
lower cylinder an eccentric movement with a cam pushing it into
place at the right time. Apparently a prototype of the second design
was built.